Method of separating and purifying gases



Aug. 11, 1936. L s. TWOMEY 2,050,511

I METHOD OF SEPARATING AND PURIFYING GASES File M y 9, 1954 5Sheets-Shet l LEE TWOMEY Nl ENTOR RNE Y Lfs. TWOMEY 2,050,511

METHOD OF SEPARATING AND PURIFYING GASES iled M y 1934 5 Sheets-Sheet 2Aug. 11, 1936.

3 3 2 71 :Tm QT: v 81 u! mm 1 LEE S.TWOMEY p VVNVENTOR /M M,

- A RNEV Aug. 11, 1936. 1.. s. TWOMEY 2,050,511

METHOD OF SEPARATING AND PURIFYING GASES Filed May 9, 1934 3Sheets-Sheet 3 EFFECTS O F TEMPERATURE A1311) PRESSURE Q N BLUE G A SFREED FROM CQZ CONTAINING HYDROGEN I. %CARBON MONOXIDE 4 5. 3

. NITROGEN 3J7.

EFFECT PRODUCIBLE B Y METHANE COOLING.

PRESSURE ON EVAPORATING METHANE L36 ATM I 1.0 ATM. I 0.5 ATM. I 0.2ATM.4I 1 TEMRERATURE OF EVAPORATING METHANE COMPRESSION 116'K I 112 K I104' K I 95' K I PRESSURE 0N BLUE GAS NITROGEN AND CARBON MONOXIDE INPRODUCT HYDROGEN.

' 3.1% 3.1% 31 34 NITROGEN ATM.

45.3% 45.3% 45.3 36.0 CARBMON;

3.1% 3.1 35 4.3 NITROGEN ATM.

45.3 45.3 35.0 13.0 I CARE. MoN.

3.1 3.3 4.0 4.1 NITROGEN ATM.

45.3 40.0 23.3 12.0 CARB. M0N.

3.5 3.1 4.3 4.5% NITROGEN ATM.

31. 5 30.0 11.4% 9.07. cARB. MON.

50 ATM. 3.1 3.9 4.5% 5.0% NITROGEN 30.0 250 14.0% 1.2% CARBMON.

EFFECT PRODUCIBLE B Y NITROGEN COOLING.

I PRESSURE ON EVAPORATING METHANE 4 ATM. I 1.5 ATM. I 10 ATM. I 05 A M.I 0.2-ATMJ TEMPERATURE OF EVAPORATING METHANE COMPRESSION 91K I 31N I1114 I 12'? K I GS'K I PRESSURE 0N I BLUE GAS. NITROGEN AND CARBONMONOXIDE IN PRODUCT HYDROGEN 3.9% 4.3% 5.0 I 5.0 2.0 NITROGEN 10 ATM. q1

25.0% 10.0% 10 3.0 1.2 CARB. MON. 52 5.3 2.5 v1.0 NITROGEN 20 ATM. 1

13.0 5.0 35 1.5 0.6 cm 13. MON.

4.9 5.0 3.3 1.1 0.1 NITROGEN 30 ATM.

3.1 3.3 2.3 1.0 0.4 cARa.M0N. 5.1 1 3.0 2.5 1.2 0.5 NITROGEN 40 6.5 2.51.3 0.5 0.3 CARB.MON.

5.2 3.0 2.0 1.0% 0.3% NITROGEN 5.2 2.0 1.4 0.5% 0.2% CARB. MoN.

FIG-3 LEE 4 WOMEY 1 INVENTOR Patented Aug. 11, 1936 UNITED STATES PATENTorrica LIETHOD F SEPARATING AND PURIFYING GASES Lee S. Twomey, Vista,Calif.

Application May a, 1 34, Serial No. 724,699

' 4Claims. (Cl.183--115) The object of my invention isto provide meansand a method for separating mixtures of gases into their constituents.The general method employed includes the old and: well known step of 5cooling the gas to a temperature at which one or more of itsconstituents liquefies while at least one of the constituents remainsgaseous, followed by a separation of the liquid from the gaseousproduct. With this method I have com- '10 bined certain steps whichmaterially reduce the amount of power required to produce the coolingeffect, and other steps which increase the sharpness of the separationand permit a more complete separation of the condensible from theincondensible gases than has heretofore been possible.

While no restriction is placed on the use of the method and apparatushereinafter disclosed, it is directed primarily to the purification ofhydrogen and to the quantity production of commercially pure or ofhighly purified hydrogen from such mixtures as blue" water gas, of whichthe following is a typical analysis:

Per cent 25 Hydrogen 49 Carbon monoxide 43 Carbon dioxide 4 Methane 1Nitrogen 3 30 The following description refers to the attached drawingswhich is a diagrammatic vertical section of a suitable form of apparatusfor performing themethod steps. In this description, for the purpose ofillustration, the gas fed to the apparatus will be considered to be theblue gas above referred to.

Figs. 1 and 2 represent respectively the left and right halves of thecomplete apparatus, which may be seen as a whole by joining the sheetson 40 the line A-A.

Fig. 3 is a table indicating certain effects of temperature and pressurein changing the composition of the above blue gas.

Referring to Figs. 1 and 2 of the drawings: iii, i I, I2, and i3 arecompressors for ammonia, ethylene, methane, and nitrogen respectively.These compressors deliver their respective compressed gases into meansfor removing the heat 50 of compression and returning the several gasesto atmospheric temperature. Such means are illustrated at ll, l5, l6,and i1 as tubular coolers having inlets and outlets for water flowingaround the tubes, but any preferred form of atmospherica, temperaturecooler may be used.

These'four primary coolers deliver their respective products intosecondary tubular units i8, i9, 20, and ii, in which they are furthercooled as will be described.

Gaseous anhydrous ammonia is raised by com- 5 I pressor iii to apressure of about Iii atmospheres absolute at which pressure itcondenses in water cooler it, the liquefied gas passing through pipe 22into secondary cooler it where the temperature of the liquid is reducedbelow atmospheric by heat interchange against coldblue gas produced inthe absorber unit later to be described. As this cooling gas may not beavailable at all times, a crossover pipe" and diversion valves 24 and 25are provided for cutting out cooler l8. The liquefied ammonia, which mayor may not be supercooled, collects in a receiver 26.

From this receiver a stream of liquid ammonia passes through pipe 21 toan expansion valve 28 by which it is admitted to the shell of condenserit, which is maintained at substantially atmospheric pressure (say from0# to 5# gauge). At this pressure the ammonia vaporizes in withdrawingheat from and condensing compressed ethylene flowing through the tubes,and the resultant ammonia gas returns through pipe '29 to the .suctionof compressor Iii at substantially atmospheric temperature and pressure,thus completing the ammonia cycle.

Assuming a cooling water temperature of 20 C. 30 (293 K.) and a lag of 50. due to imperfect interchange, the liquid ammonia will collect inreceiver 26 at 298 K., at which temperature its vapor pressure is 10atm. absolute. On entering cooler l9 its temperature drops to itsboiling point at the pressure carried, being 240 K. at 0# and 247 K. at'5# gauge. If interchange It is in operation, the temperature of theammonia in receiver 28 will be lowered and the condensing pressure andthe power load on compressor iii 40 will be' reduced. The expansiontemperatures, in cooler l9, will not be altered.

Gaseous ethylene is raised by compressor ii to. a pressure of about 22atm. absolute. is brought back to atmospheric temperature in cooler i5and passed through pipe 30 into the tubes of cooler is, where it isfurther cooled and is condensed by evaporating ammonia. The liquefiedethylene passes through pipe 3i into a receiver 32. From this receiver astream of ethylene passes through pipe 33 to an expansion valve 34 bywhich it is admitted to the shell of condenser 20, which is maintainedat substantially atmospheric pressure (say from 0# to 5 gauge). At thispressure the ethylene vaporizes in withdrawing heat from ture andpressure, thus completing the ethylene cycle.

Assuming a 5 temperature lag in condenser I9, as before, the liquidethylene will collect in receiver 32 at 245 to 252 K., at whichtemperatures its vapor pressure is from 20 to 24 atm. On enteringcondenser 20 its temperature drops to its boiling point at the pressurecarried, being 168 K. at 0# and 173 K. at 5# gauge.

Gaseous methane is raised by compressor H to a pressure of about 28 atm.absolute, is brought back to atmospheric temperature in cooler l8 andpassed through pipe 36 into the tubes of cooler 20 where it is furthercooled and is condensed by evaporating ethylene. The liquefied methanepasses through pipe 31 into a receiver 38. From this receiver a streamof methane passes through pipe 38 to an expansion valve 40 by which itis admitted to the shell of condenser 2l, which is maintained atsubstantially atmospheric pressure (say from 0# to 5# gauge). At thispressure the methane vaporizes in withdrawing heat from and condensingcompressed nitrogen flowing through the tubes, and the resultant methanegas returns through pipe 4| to the suction of compressor l2. Thiscompletes the direct methane cycle, but there is a secondary cycle inwhich methane is withdrawn from the receiver and returned to thecompressor for the intermediate cooling oi the blue gas. as will bedescribed.

Assuming 8, 5 temperature lag in condenser 20, the liquid methane willcollect in receiver. 38 at from 173 to 178 K. at which temperatures itsvapor pressure is from 25 to 30 atm. On entering condenser 2| itstemperature drops to its boiling point at the pressure carried, being112 K. at 0# and 116 K. at 5# gauge.

Gaseous nitrogen is raised by compressor Hi to a pressure of about 25atm. absolute, is brought back to atmospheric temperature in cooler l1and passes through pipe 42 into the tubes of cooler 2| where it isfurther cooled and is condensed by evaporating methane. The liquefiednitrogen passes through pipe 43 into a receiver 44. From this receiver astream of liquid nitrogen is withdrawn through pipe 45 for use inanother part of the system, as will be described. Assuming a 5temperature lag in condenser 2|, nitrogen will collect in receiver 46 atfrom 117 to 121 K at which temperature its vapor pressure is from 22 to27 atm.

Describing now the flow oi blue gas (or other gas to be treated) throughthe apparatus: raw

gas at atmospheric temperature and pressure enters the system throughpipe 46 and is raised by a compressor 41 to a pressure of, for example,20

atm. absolute. The pressure at the ultimate point of pressure releasemay be of the order of 20 atm. plus pressure drop through the apparatus.As this pressure drop willvary with details of construction and withoptional variations in operating method, the compressor pressure cannotbe specified, and the release pressure of Ali: aim. is a preference onlyand may be widely depart-ed The compressed gas passes through the tubesof a water cooler 48 in which the heat oi compression is removed and thegas brought bacl: to atmospheric temperature, and thence through pipe 49into the shell of a dehydrating interchanger iii. If desired, thecompressed gas may be passed through scrubbers or other means forremoving any carbon dioxide which may be present in the raw gas beforeentering the interchanger.

This interchanger is of the well known horizontal type consisting of abundle of tubes through which the cooling medium passes, these tubesbeing enclosed by a shell through which the blue gas passes. This shellis divided into a series of pockets by staggered baflles 5|, by whichthe length of travel of the gas and its velocity over the tubes areincreased, each pocket so formed being drained as required by means of avalve 52.

The tubes are divided into groups by means of headers at each end of theshell, the headers shown being concentrically arranged as indicated at53, 54, 55, and 66, and through these groups of tubes are passed variousgases which have been chilled and partially rewarmed in later stages oi.the operation. The tem erature of these gases is such that the blue gas,leaving the interchanger, is reduced to a temperature below the freezingpoint of water, and as this unit gradually becomes choked by theaccumulation of ice, it should be provided in duplicate and with properdiversion pipes and valves to permit one unit to be thawed out while itsduplicate is in operation.

50 and passes through pipe 51 into the shell of an interchanger 58 whichmay be of the same general construction as interchanger 50 except thatthe water drains 52 are not required. In this interchanger it is furthercooled against various gases which have been chilled and partiallyrewarmed in later stages of the operation, and also by the evaporationof liquid methane.

In this interchanger the gas may be cooled to such point that the carbonmonoxide content is partly condensed. This cooling efiect is produced byinterchange and by the evaporation of liquid methane which is drawn fromreceiver 38 through pipe 60 and .an expansion valve 6| into one of thetube groups of the interchanger, which is maintained at or belowatmospheric pressure. At these pressures the liquid methane vaporlzes inwithdrawing heat from the blue gas, thus causing the condensation ofcarbon monoxide. The resultant methane gas passes through pipe 62 to atube group of dehydrating interchanger 50 and passes finally throughpipe 63, at substantially atmospheric temperature and at or belowatmospheric pressure, to pipe 4| and thus to the suction of compressorl2. v

At 0# gauge pressure methane boils at 112 I v The dehydrated blue gasleaves interchanger K., at 5# gauge at 116 K. With care in the design ofinterchanger 58 the eiiiuent gas may be cooled to within 2 C. of thesetemperatures, or from 114 K. to 118 K. At these temperatures the vaporpressures of carbon monoxide are 13 atm. and 17 atm. respectively, andat an operating pressure of 20 atm. no condensation of carbon monoxidein the gas entering the condenser exceeded 65% and 85% respectively. Athigher compressions and at lower back pressures on the compressor theconcentration of carbon monoxide in the gas delivered by interchanger 58would be as shown by the table in Fig. 3.

As it may under some conditions be undesirable to pass the entirecondensing load to the final interchangers 16, 18, and and may also beundesirable to largely increase the compression of the blue gas, Iprefer in such cases to introduce a secondary exhauster into the courseof pipe it. Such exhauster is indicated at M, a branch suction ii and abranch discharge 8! being connected into pipe I on opposite sides "of astop valve 81. Valves," and provide for cutting out the exhauster whenvalve 61 is opened to connect compressor l2 direct to interchanger Bil.The use of an auxiliary exhauster provides for any desired reduction intemperature'otinterchanger 6| without operating compressor ii at adiminished back pressure, it being assumed that the exhauster willdischarge at a pressure equal to the desired suction pressure on methanecompressor l2.

From interchanger it the cooled blue gas,

which may contain more or less condensate of carbon monoxide. passesthrough pipe 69 to a mist extractor 10 which may be 0! any preferredform. As illustrated, the upper portion or the shell is filled withbroken coke or other fragmental or shaped inert material "ii resting on'a screen or perforated plate I2. Thisdevlce, which is well known, ishighly effective for the separation of the line mist oi condensate whichforms on cooling the gas to the condensing point oi carbon monoxide.

The liquid thus separated iromthc gas collects in a pool ii in thebottom of the extractor while the gas is withdrawn from beneath a shield14 and passes through pipe Ii to the shell of an interchanger 18 inwhich it is eounterflowed against cold gaseous nitrogen and hydrogenproduced by succeeding operations and is cooled by evaporating carbonmonoxide, as will be described. This interchanger is provided withbatfles and may be of the same general form as interchanger 58. It thenpasses through pipe ll to theshell of an interchanger II in which it iscounterilowed against and cooled by gaseous hydrogen and evaporatingliquid nitrogen, and then through pipe 19 to an interchanger Iii inwhich it is counterflowed against and cooled by expanding gaseoushydrogen. By this successive cooling it is reduced to a temperature atwhich most of the carbon monoxide is condensed and largely in the formof a mist suspended in the residual hydrogen.

- This mist passes through pipe II to a second mist extractor 82, inwhich the liquid collects as a pool 08. The liquid carbon monoxide,together with some nitrogen which may condense with it, collects inpools I! and It and is withdrawn through a branchedpipe 84 provided withdiversion valves CH6 and an expansion valve 81 by which it is admittedto one of the tube groups. oi interchanger IO. This space beingmaintained at approximately atmospheric pressure, the mixed carbonmonoxide-nitrogen liquid evaporates and is reduced to approximately itsboiling point at l atm. absolute or 82 K, and I the blue gas passes out01' this interchanger at a temperature 1 or 2 C. higher.

From interchanger I. the gaseous carbon monoxide passes through pipe llto interchanger 58 where it is counterilowed against compressed andpartially cooled blue gas, then through pipe 8! to the dehydratinginterchanger. I against compressed blue gas, finally leaving the systemthrough pipe 90 at substantially atmospheric temperature and pressure.

One of the groups of tubes of interchanger ll is supplied with liquidnitrogen drawn from receiver 44 through pipe 45 which is provided withan expansion valve 8|. This space is maintained being provided withvalves ill-I22. It will be at a pressure which will vary with thecooling eii'ect available from the expansion of the residual hydrogen inthe same interchanger, thiseflect being variable with options as to theoperation 0! further parts of the-system. The minimum pres- 5 i sure inthis group of tubes will ordinarily beat the order 01' 2/ atm. absolute,at which nitrogen boils at 66 K., the freezing point of carbon monoxide.and it may be very much higher. The expanded gaseous nitrogen passesthrough pipe 92 to interchanger I6 then through pipe 93 to interchanger68, then through pipe 94 to dehydrating interchanger ill, and throughpipe 95 to the suction side of compressor It at substantiallyatmospheric temperature.

The gas removed from extractor 02 passes through pipe 98 intointerchanger 80 and enters the tubes in an expanded condition andconsequently at a temperature below that at which it is freed fromcondensate in separator 82. This expansion is produced-at an expansionvalve 81 (valve I", being open) in-case stop valves II and I" in thepipes leading to the absorber set are closed and that apparatusinoperative. By interchange in this unit the ilnal temperature of theblue gas entering separator 82 is so reduced as to condense iurtherquantities of carbon monoxide as already described.

The hydrogen passing through the tubes oi interchanger Bil passesthrough-pipe as to interchanger It, then through pipe IM to interchangerIt, then through pipe "ii to interchanger 58, then throughpipe I02 todehydrating interchanger BB, and is finally withdrawn from the systemthrough pipe [03 at substantially atmospheric temperature and pressureas the purified hydrogen product. The degree oi purity will vary withcertain options as to manipulation, not yet described.

Instead of taking the product hydrogen direct from the expansion step itmay be submitted to contact with solid adsorbents by which the highestdegree oi. purity .is produced.

Describing first the absorbers. these are indicated in the drawings asvertical cylinders numbered I08 to Ill) inclusive. These cylinders arefilled with a desired solid absorbent, such for example as activatedcharcoal, this char resting on a screen or'other support ill. Thesesupports, and appropriate il'iling and discharge manholes H2 and H8 areshown on one cylinder only, but all are equipped in the same manner.

The live absorbers are connected in series by pipes numbered 4 to illinclusive, these pipes noted that pipe ill from the bottom oi cylinderilli leads to the top oi cylinder lot; so that the series is cyclic.

Around each '0! these valves is connected a shunt pipe Iii-I28 providedwith a valve having the same number, and in each of these pipes isplaced an oriilce fitting I294" having means for adjusting or changingthe size of the orifice.

A pipe I is connected to cold hydrogen outlet pipe I. and is branched atIla-I39 to admit 1 hydrogen to the top or any one of the absorbers, eachbranch being provided with a valve of the same number. 7

A. pipe I is extended from a connection with dehydrated blue gas pipe 51to anlnterchanger ill and irom this interchanger an outlet pipe I isbranched at ill-I45 to admit blue gas into the top of any one absorber,each branch having a valve. A rotary blower or other low head gas pump Imay be placed in pipe I40 7 to urge the requisite gas supply to andthrough the absorber.

A pipe I" provided with a valve III is branched from pipe 00 and isitself branched at Ill-I02 to permit cold hydrogen to fiow from thelower end of any cylinder into interchanger 80. For diverting the gasleaving separator 02 through pipe I to an absorber and back tointerchanger 00, pipe 00 is provided with a valve Ill between the twobranches. v

Each of the branches I ll-IH is further provided with an adjustableorifice fitting Ill-Ill as above described.

A pipe I00 is connected to the upper end of the shell of ammoniainterchanger I0. and is branched at IOI-I0l to permitblue gas 'used inheating to be withdrawn from the bottom of any one cylinder, each ofthese branches being provided with a valve. From the lower end ofinterchanger I0 a pipe I60 conducts the warmed gas to interchanger I05from which it fiows through pipe IG'I to the blue gas inlet ofinterchanger 58.

A pipe I00 is branched at- "9J1: topermit warm hydrogen to be withdrawnfrom any one of the absorbers, each branch having a valve of the samenumber. Purified hydrogen is vented from the system at substantiallyatmospheric temperature and either at system pressure or atmosphericpressure as may be preferred.

It is well known that when a mixture of two gases or vapors havingdifferent liquefying temperatures at any given pressure is contactedwith certain solid adsorbents, the more readily condensible constituentis selectively'adsorbed by the solid and may thus be partially orcompletely removed from the less readily condensilele. It is also wellknown that the adsorption of the more readily condensible matter and itsretention in the adsorbent solid is facilitated to some extent byincrease in pressure and, usually to a greater extent, by decrease intemperature, and that the adsorbed matter may be removed from the solidadsorbent by heating it to a temperature materially above the boilingpoint of the adsorbed matter at the existing pressure.

This is commercial practice in the separation of liquid hydrocarbonsfrom natural gas by passing the gas, usually at substantiallyatmospheric temperature and pressure, through columns of adsorbentcharcoal and intermittently removing the absorbed liquids from the charby heating it with direct steam.

In the present invention I have applied these well known principles tothepurification of a gas (as for example hydrogen) from gases havinghigher boiling points (as for example nitrogen and carbon monoxide) bythe provision of means for cooling the mixed gases to very lowtemperatures, of means for removing the adsorbed impurities from thecharcoal, of means for precooling the absorbers and of means forregenerating the extremely low temperatures employed without losing anyof the cooling effect residing in the cold intermediate or finalproducts. By these means the known theories of selective adsorption aremade available for the commercial puri= fieation and separation of theso-called fixed" gases at reasonable costs, an end not heretoforeattained.

Before describing the operation of the absorber unit it should bepredicated that the operation is euentially intermittent and that eachcylinder is used successively in three operative stages, the startingoint of th operation passing from one cylinder to the next in order asrepresented 'in the drawings, reading to the right.

The three stages of the operation are as follows: first, absorption, inwhich clean, precooled charcoal is contacted with a fiow of the gas tobe 5 purified, this stage terminating when the charcoal has becomesaturated with impurities; second, the cleaning of the charcoal. hereintermed heating in which the saturated char isheated to such temperaturethat the impurities are gasified and driven on; third, precooling, inwhich the clean-char and its container are brought back to thetemperature of the gas flow and thus fitted for reuse in the firststage.

' Flor reasons which will appear, it is preferable 1 to utilize at leasttwo cylinders in series in each the first and third stages, and in thefollowing description we will assume that cylinders I08 and I01 are inthe first or absorbing stage, cylinders I08 and I09 are in the third orprecooling stage, and cylinder I I0 is in the second or heating stage.It will be understood that in the description following the three stagesof absorption, heating. and cooling occur simultaneously in the group ofabsorbers and successively in any one, and that as soon as one stage iscompleted in any one absorber, that unit is ready to be moved up to thebeginning of the next stage. All valves are assumed to be closed exceptwhen stated to be open.

Starting from separator 82, in which a continuous supply of coldhydrogen containing more or less carbon monoxide and nitrogen isavailable, and closing valve I59 to prevent this hydrogen from passingto interchanger 80, we open valve 98 allowing the hydrogen to pass underpressure through pipe I to valve I35 and thus into the top of cylinderI08. In this cylinder the nitrogen and carbon monoxide are at leastpartially absorbed from the hydrogen, which passes through pipe I! andvalve M9 to the top of absorber I01, in which any impurity escaping fromabsorber I08 is absorbed. While this operation is in progress, cylindersI00 and I09, which have been used and cleaned, are in progress ofprecooling and cylinder H0 is in progress of heating and cleaning, aswill bedescribed.

Unless the carbon monoxide content of the entering gas be very large thequantity of cold gas which may be passed through a single absorberbefore it becomes completely charged with impurities is much greaterthan the quantity required to cool a cleaned absorber from atmospherictemperaturev to the temperature of the cold gas.

As in the system here described, the gas which has passed through twoabsorbers is used to directly cool the next two cylinders, valve I20 inthe direct line between cylinders I01 and I08 is left closed, but shuntvalve I25 is opened, permitting gas to fiow through orifice I30 intocylinder #00. At the same time valve I is opened permitting gas to fiowthrough orifice I to the cold hydrogen discharge pipe I". These orificesmay he so proportioned in area that the quantity of cold 88s passed intocylinder I08 is Just sufilcient to cool one cylinder in the minimum timerequired to charge one cylinder with impurities, and ii in practice agreater time be taken to exhaust the absorbing cylinder, the shunt valvemay be closed when cylinder I00 is completely cooled.

The fiow of cold gas passing through orifice I30 to cylinder I00 iscarried into cylinder I00 through valve III and pipe III, and is finallyremoved from this c linder through pipes I11 and its issuing from thelatter as highly purified hydrogen at atmospheric temperature andpressure, the release from system pressure to atmospheric occurring inorifice I30.

the hydrogen leaves separator 82. It is possible to release the.pressure at valve- 88 instead of at the orifices, in which case theabsorption is conducted at atmospheric pressure. This produces a lowertemperature in the absorber than that available at high pressure andpermits the use of absorbers designed for low pressures.

The use of cylinder I08 as the first absorber may be continued untiltraces of impurities begin to show at the outlet of absorber I01 or cometo.

some predetermined proportion.

Simultaneously with the absorption in cylinders I08 and I01, cylindersI08 and I08 are being precooled and made ready for absorption.

The cold purified hydrogen passing through orifice I30 and pipe I'ISinto the top of cylinder I08 completes the cooling of the char in thiscylinder and passes on through valve I2I and pipe II8 to cylinder I09,in which the precooling begins, this cylinder being the last to come outof the previous heating stage. From cylinder I08 the hydrogen passes outof the system as purified product hydrogen at substantially atmospherictemperature and pressure through valve I12 and pipe I88.

Simultaneously with absorption in I08 and I01, cylinder IIO, which hasbecome completely saturated in a previous cycle, is being heated for thepurpose of cleaning. Blue gas is suitable for this purpose but it is notdesirable to use the raw gas entering the system as it usually containsso much water vapor as to cause the accumulation of frost in the coldsaturated charcoal.

We therefore start from pipe 81, which is constantly supplied withpartly cooled and substantially dehydrated blue gas andpass this gasthrough pipe I04 to an interchanger I08 in which it is counterfiowedagainst a warm gas and is brought up to atmospheric temperature. Fromthis interchanger the blue 1 gas passes through blower I48 and pipe I40to branch pipe and valve I48 and into cylinder 0, which has Just comeout of the absorbing stage of the previous cycle. I

From the bottom of H0 the blue gas is directed through pipe and valveI88 and pipe I80 to ammonia interchanger I8, in which any cooling effectdue to subatmospheric temperature of the gas is useful for subcoolingliquid ammonia and reducing the power load on compressor I0.

From this interchanger the warm gas, carrying the impurities removedfrom cylinder IIO, passes through-pipe I68 to the lower endofinterchanger I08 where it counterfiows the cold blue gas from thedehydrating interchanger 80. and returns from interchanger I08 tointerchanger 88 through pipe I81.

If desired. warm product hydrogen from pipe I08 may be used to replacewarm blue gas in the above heating operation but asthis step causes ,thecontamination of the hydrogen, it,

would be necessary to return it to compressor 41 for retreatment andwould thus reduce the pure hydrogen output.

. The blue gas removes the impurities from the char in cylinder IIO byheating it to about atmosp'heric temperature, which is far above thecritical temperatures of carbon monoxide and nitrogen. gas. issuing fromcylinder H0 is at first at the adsorption or minimum temperature, andthe heating stage is complete when the temperature of the gas has risento atmospheric. As this gas of variable temperature is interchangedagainst a stream of liquid entering the interchanger (I8) at the maximumtemperature of the.gas, any heat absorbing capacity which it may possessat subatmospheric temperature is utilized in. precooling the ammonia'andreducing the average temperature of the contents of receiver 28, whichshould be of such capacity as not to permit its contents to fluctuate intemperature to any objectionable degree by reason During the heatingperiod the blue of the variable temperature of the stream of ammoniaentering it from interchanger I8,

The quantitative cooling effect of the returned blue gas cannot bepredicted, except for a specific-case, as it varies over a very widerange with variations in the proportion of impurities in the gas, therate at which the cold absorber is heated and other factors. It may begreater than the ammonia stream required for refrigeration can absorbwithout freezing, and in such case it would be desirable to insert aninterchanger similar to I8 in the ethylene cycle, as between l8 and 82,or even into similar positions in the ,methane and nitrogen cycles, thusgradually raising the temperature of the cold gas and distributing thecooling effect over a succession oi refrigerants of increasing freezingpoints.

The heating stage terminates when absorber I I0 is brought toatmospheric temperature at its lower end, at which time it is clean andready for precooling.

When the three stages are (more or less simultaneously) completed, thesequence is moved one step to the right. That is to say, cylinder I08,which is now completely charged with impurities, is transferred to theheating stage, cylinder I08, which is now completely cooled, becomes thesecond absorber, and cylinder IIO, which is now completely heated,becomes the second cylinder in process of cooling. To effect thesechanges the following valve manipulations are made.

Divert'cold hydrogen from I08 to I 01 by closing I38 and opening I88.

Divert cold hydrogen outlet from I01 to I08 by closing I48 and openingI80.

Divert point of pressure release from I01I08 to I08I08 by closing shuntvalve I28 and opening shunt valve I28, also closing main valve H9 siblewith a single cylinder. As the char prograsses toward saturation, itsadsorption rate is progressively lowered and it is impossible tocompletely utilize the adsorptive value of a single body of char withoutvery greatly lowering the flow rate of the gas being treated. The secondadsorber, containing fresh char, effectively removes all impuritiespassing from the first cylinder as it approaches saturation, and permitsthe utilization of the entire adsorptive value of the first char bodywithout retardation of the gas fiow rate.

The desirability of using at least two cylinders in series in theprecooling stage is based on somewhat difi'erent grounds The existingconditions-direct contact of gas with the char, relatively high heatconductivity of the char and absence of convection in the interstices ofa pack of granular solids-are ideal for rapid heat interchange in eitherdirection between the solid and the gas. For this reason the coolingeffect takes place, not through the entire length of the cylinder atonce, but in a zone of relatively small depth which progresses throughthe cylinder which has been cooling for say half the time required forcompletion, the upper portion of the body of char is completely cooledto the minimum temperature, the lower portion is at its original (themaximum) temperature, and an intermediate zone is in progress ofcooling, at the minimum temperature on its upper side and at the maximumtemperature below.

The depth of this zone will vary with the conductivity of the char, thevelocity of the cooling gas, the extent to which channeling occurs andother variables. It will be evident that, no matter what its depth, asingle cylinder will discharge a gas of falling temperature during thetime required for this zone to travel through the cylinder for adistance equal to its depth: 1. e., the time required for the zone topass out of the cylinder. It is equally evident that if the depth ofthis zone does not exceed the depth of the second absorber, the zone ofchanging temperature may be moved to such position in this second unitthat the gas issuing from the first will be at one extreme of thetemperature range while that issuing from the second is at the otherextreme, thus providing for the complete cooling of the first body ofchar without discharging any gas of intermediate temperature.

In case the carbon monoxide content of the blue gas is so low that nocondensate is producible by applying the methane cooling step to theblue gas under the operating conditions here described, this step may beomitted by cutting 01! the supply of methane to expansion valve 8|. Asmethane refrigeration is materially cheaper than that produced by liquidnitrogen, it is desirable to utilize the higher temperature cooling inso far as it may be available in any specific case, and in such case thebalance of cost between higher blue gas pressures and/or reducedexpansion pressures on the methaneon the one hand and an increasednitrogen consumption on the other will be a matter for calculation.

In some cases it may be desirable to omit the nitrogen cooling step.When operating in this manner, the methane interchanger l! is maintainedat a-pressure of say 0.2 atm. absolute, at which methane boils at 95 K.and an outlet temperature of' about 98 K. may be obtained. At thistemperature the vapor pressure of carbon monoxide is 4.8 atm. absolute ad 8 :8

containing more than 9.6% of carbon monoxide would liquefy all excessover that quantity. The excess would have to be considerable to providefor the functioning of the next step, the reduction of the temperatureof the gas to the boiling point of liquid carbon monoxide ininterchanger 16. Assuming that the cooling efiect of the expandedhydrogen would at least offset the lag in carbon monoxide interchange,the temperature of the gas entering separator is 82 K. at which thevapor pressure of carbon monoxide is 1 atm. absolute and the proportionof carbon monoxide in the eilluent gas at 20 atm. pressure is 5.0%.

This degree of fractionation is obviously of no value when considered asa means of purifying hydrogen, and as carbon monoxide has little presentvalue other than as a fuel, this step alone would be without purpose.Under some conditions, however, it might be desirable in connection withthe absorption step, as while the load thrown on the absorbers would bemany times increased (in the ratio of 0.11 at 66 K. to 1.0 at 82 -K.),the absorption step is relatively inexpensive as regards powerconsumption and maintenance and it might well be more economical in someinstances to operate under an increased absorber load and a muchdecreased load on-the refrigeration cycles.

I claim as my invention:

1. The method of separating a mixture of gases into desired andundesired fractions which comprises: cooling a stream of said mixture toa temperature at which the undesired fraction is partially condensed andat which the desired fraction remains gaseous; separating the condensateformed by said cooling from said stream; passing said streamsuccessively through a first and a second permeable body of solidabsorbent material, said bodies being precooled to a temperature notsubstantially above said condensation temperature; absorbing andretaining fur ther quantities of the undesired fraction in the absorbentmaterial of said first and second bodies, whereby the desired fractionis purified; dividing the efliuent stream from said second body to formtwo lesser streams of said purified fraction; passing one of said lesserstreams in heat interchange relation with said mixture stream to assistthe cooling of said mixture and discharging said lesser stream from thesystem at substantially atmospheric temperature; passing the other ofsaid lesser streams successively through a third and a fourth permeablebody of solid absorbent material, said bodies being initiallyapproximately free from the undesired fraction and at substantiallyatmospheric temperature; continuing the passage of last said lesserstream until said third body is cooled to substantially saidcondensation temperature and discharging last said stream from saidfourth body at substantially atmospheric temperature.

2. A method substantially as and for the purpose set forth in claim 1,in which said mixture stream is raised to a material superatmosphericpressure prior to said cooling and said pressure is released tosubstantially atmospheric at substantially the point of eiiiux of saideilluent stream from said second permeable body.

3. The method of separating constituents of a mixture of gases whichcomprises: cooling a stream of said mixture to a low subatmospherictemperature; passing a stream of said mixture through a precooled firstbody of solid absorbent material and absorbing and retaining in saidbody a constituent of said mixture; dividing said stream, withdrawingone portion-of said stream from the system and utilizing said portion infirst said cooling of said stream; passing the remaining portion of saidstream without avoidably changing its temperature through a succeedingplurality of bodies of solid absorbent material initially at highertemperatures, whereby said bodies are precooled bythe heat absorbingcapacity of said remaining portion, and discharging said remainingportion from the system at a substantially constant higher temperature.

d. A method substantially as and for the purpose set forth in claim 3,in which one portion of said divided stream is passed through apluralityoi initially warmer bodies in series, in which said passage iscontinued until the first body of said series is reduced to the desiredtempera ture of precoolihg, and in which the first body whensufiiciently precooled is withdrawn from said series and an additionalwarm body is added to said series at the discharge end thereoi.

LEE 5. TWO.

